The influence of slab-derived sulfur on the sulfur content and oxidation state of arc magmas in the Southern Cascades

Abstract:

Arc magmas are oxidized relative to mid-ocean ridge basalts, but the cause of this oxidized signature is debated. Some evidence indicates that arc magmas evolve towards more oxidizing conditions during storage in the crust, while other evidence points toward the influence of oxidizing slab-derived material in the mantle source. Slab-derived sulfur has the potential to be a powerful oxidant in the mantle wedge, yet the cycling of sulfur through subduction zones is poorly understood.

Here we track the influence of slab-derived sulfur on the sulfur content and redox state of arc magmas using primitive olivine-hosted melt inclusions from cinder cones within Lassen Volcanic Area in the Southern Cascades. We combine measurements of major and trace elements, S⁶⁺/ΣS, Fe³⁺/ΣFe, δ³⁴S, and volatile contents to create a model of sulfur cycling in the Southern Cascades and characterize its relationship to arc magma redox state. Estimated primary magma S contents from cinder cones range from 790 to 1940 ppm. S⁶⁺/ΣS ranges from 0.36 to 0.99, and estimated primary Fe³⁺/ΣFe ranges from 0.15 (QFM + 0.3) to 0.28 (QFM + 2.1). Measured δ³⁴S ranges from 0.8 to 5.5‰. Each of these parameters increases with Sr/Nd, a proxy for the proportion of slab melt added to the mantle source. These correlations demonstrate that measured sulfur at Lassen is partially sourced from the subducting slab and that the influence of slab-derived sulfur is tied to arc magma redox state. This relationship is consistent with evidence from previous work [1,2] that the addition of slab-derived material creates volatile-enriched calc-alkaline magmas at Lassen.

To quantify the relationship between slab-derived sulfur and arc magma redox state, we combine pMELTS models, trace element partitioning, coupled S and Fe redox, and S isotope mass balance calculations to model the influence of oxidized sulfur on the sub-arc mantle. We find that slab-derived sulfur can oxidize the mantle source of arc magmas, but oxidation caused by sulfur enrichment requires an unusually sulfur-rich slab melt and/or multiple stages of melt fluxing in the mantle source. These modelling results are strong evidence for a causal link between slab-derived sulfur and the oxidation state of arc magmas in the Southern Cascades.

[1] Walowski et al. 2016, EPSL. [2] Borg and Clynne 1997 Can. Min.